1. Field of the Invention
The present invention relates to a photo-detecting apparatus that removes a background light component of incoming light incident on photodetectors to detect only a signal light component.
2. Related Background Art
The photo-detecting apparatus has one or more photodetectors, and integrates signal currents output from the photodetectors by an integrating circuit and outputs the integrated result as a signal voltage. Some photo-detecting apparatuss convert the signal voltage into a digital signal (A/D conversion) and output the converted digital signal. If the signal voltage exceeds a predetermined value during the A/D conversion, a problem arises that the digital signal which was A/D-converted from the signal voltage is set to a value corresponding to that predetermined value or becomes saturated, failing to perform accurate light detection. To deal with this problem, a conventional practice involves setting the predetermined value to or higher than an estimated maximum value of the signal voltage to prevent the saturation. Another method uses a logarithmic compression technique to expand a dynamic range.
The PHOTO-DETECTING APPARATUS is used in a range sensor installed into a camera, for example. In this range sensor the reflection of a spot light thrown onto an object from a light emitting means such as light emitting diode is detected by two photo-detecting apparatuss that produce two detection signals, based on which the distance to the object is measured. When a spot light component (signal light component) is detected, a background light component superimposed on it is also detected. To cope with this problem, only the background light component is detected when the spot light is not projected, to determine a difference between the two components and obtain a signal of only the spot light component, thereby improving the range-finding precision.
The integrating circuit in the conventional photo-detecting apparatus, however, is taken no countermeasures against noise components whose magnitudes vary from one integral operation to another, such as thermal noise produced by amplifiers, the constitutional circuits of the integrating circuit. As a result, noise-induced errors may occur. Thus, when the light intensity detected by the photodetectors, i.e., a value of the signal voltage, is small, the noise components that vary from one integral operation to another degrade the S/N ratio of light detection.
Further, in the A/D conversion in the conventional photo-detecting apparatus, because the predetermined value is set to a large value to keep the saturation from taking place, when the light intensity detected by the photodetectors, i.e., the signal voltage, is small, the resolution of the output digital signal deteriorates.
Further, the technique of determining only the spot light component by subtracting the detection result of the background light component from the detection result of both the spot light component and the background light component, as when the photo-detecting apparatus is used in the range sensor, poses the following problem. That is, when the background light component is large compared with the spot light component, the signal voltage of the spot light component superimposed with the background light component is very large, which makes it necessary to set the predetermined value to a still larger value in order to prevent the saturation. Therefore, the digital signal, which is output based on the spot light component obtained as a result of subtraction, makes the resolution even worse.
As described above, the conventional photo-detecting apparatus has a poor S/N ratio and, when the signal voltage is A/D-converted, the resolution of the output digital signal is low. Under these circumstances the present invention has been accomplished to overcome the aforementioned problems and provide a photo-detecting apparatus with an excellent S/N ratio. Another object of this invention is to provide a photo-detecting apparatus which, during the A/D conversion process, does not cause saturation if the incident light intensity is large and provides an excellent resolution if the incident light intensity is small.
The photo-detecting apparatus of this invention comprises: (1) a photodetector to output a signal current corresponding to an incident light intensity; (2) an integrating circuit to store an electric charge corresponding to the signal current output from the photodetector and output a signal voltage corresponding to the amount of stored charge; (3) a first CDS (Correlated Double Sampling) circuit having a first coupling capacitor and a first amplifier both connected serially in that order between its input terminal and output terminal, the input terminal being adapted to receive the signal voltage output from the integrating circuit, a first integrating capacitor parallelly connected between an input and an output of the first amplifier, and a first switch means for storing in the first integrating capacitor an amount of electric charge corresponding to a change in the signal voltage; (4) a second CDS circuit having a second coupling capacitor and a second amplifier both connected serially in that order between its input terminal and output terminal, the input terminal being adapted to receive the signal voltage output from the integrating circuit, a second integrating capacitor having a capacitance value equal to that of the first integrating capacitor and parallelly connected between an input and an output of the first amplifier, and a second switch means for storing in the second integrating capacitor an amount of electric charge corresponding to a change in the signal voltage; and (5) a difference calculation circuit to determine a difference between the amounts of charges stored in the first integrating capacitor of the first CDS circuit and in the second integrating capacitor of the second CDS circuit and output a signal voltage corresponding to the difference.
In this photo-detecting apparatus, a signal current corresponding to an incident light intensity is output from the photodetector, and the integrating circuit stores an electric charge corresponding to the signal current output from the photodetector and outputs a signal voltage corresponding to the amount of the stored electric charge. In the fist CDS circuit, the signal voltage output from the integrating circuit is entered into the first coupling capacitor and the amount of electric charge corresponding to a change in the input signal voltage is stored in the first integrating capacitor by the first switch means. Similarly, in the second CDS circuit, too, the signal voltage output from the integrating circuit is entered into the second coupling capacitor and the amount of electric charge corresponding to a change in the input signal voltage is stored in the second integrating capacitor by the second switch means. Then, the difference calculation circuit determines a difference between the amount of charge stored in the first integrating capacitor of the first CDS circuit and the amount of charge stored in the second integrating capacitor of the second CDS circuit, and outputs a signal voltage corresponding to the difference.
Further, the photo-detecting apparatus of this invention further includes a timing control circuit for controlling the operations of integrating circuit, first CDS circuit, second CDS circuit and difference calculation circuit, and is used along with a light emitting means for throwing a spot light toward an object. In this photo-detecting apparatus the timing control circuit performs the steps of: (1) in a first period when the light emitting means is throwing the spot light onto the object, storing a first amount of electric charge in the first integrating capacitor of the first CDS circuit based on the change in the signal voltage output from the integrating circuit when the photodetector detects the spot light component and a background light component; (2) in a second period when the light emitting means is not throwing the spot light onto the object, storing a second amount of electric charge in the second integrating capacitor of the second CDS circuit based on the change in the signal voltage output from the integrating circuit when the photodetector detects the background light component; and (3) in a third period following the first and second periods, having the difference calculation circuit calculate a difference between the amount of charge stored in the first integrating capacitor of the first CDS circuit and the amount of charge stored in the second integrating capacitor of the second CDS circuit and then output a signal voltage corresponding to the difference from the difference calculation circuit.
In this case, the photo-detecting apparatus under the control of the timing control circuit performs as follows. In the first period, when the photodetector detects the spot light component and the background light component, the first amount of electric charge corresponding to a change in the signal voltage output from the integrating circuit is stored in the first integrating capacitor of the first CDS circuit. In the second period, when the photodetector detects the background light component, the second amount of electric charge corresponding to a change in the signal voltage output from the integrating circuit is stored in the second integrating capacitor of the second CDS circuit. In the third period, the difference calculation circuit determines a difference between the amount of electric charge stored in the first integrating capacitor of the first CDS circuit and the amount of electric charge stored in the second integrating capacitor of the second CDS circuit, and outputs a signal voltage corresponding to the difference. The signal voltage output from this difference calculation circuit corresponds to the spot light component. Either of the first and second periods can occur first.
Further, the photo-detecting apparatus of this invention further includes: (1) N sets (Nxe2x89xa72) of photodetector, integrating circuit, first CDS circuit, second CDS circuit and difference calculation circuit; (2) N hold circuits corresponding to the respective N difference calculation circuits and adapted to hold and output the signal voltages output from the difference calculation circuits; and (3) an A/D converter to successively input the signal voltages output from the N hold circuits, convert the signal voltages into digital signals and output the converted digital signals.
In this case, there are provided N sets of photodetectors, integrating circuit, first CDS circuit, second CDS circuit and difference calculation circuit. The signal voltage output from the difference calculation circuit of each set is held in the hold circuit. Then, the A/D converter successively receives signal voltages from the N hold circuits, converts the signal voltages into digital signals, and outputs the converted digital signals. That is, when a one- or two-dimensional image is photographed, the photographed result is output as the digital signal.
The photo-detecting apparatus of this invention further includes a maximum value detection circuit to detect a maximum value of the signal voltages output from the N difference calculation circuits or the N hold circuits and is characterized in that the A/D converter sets an A/D conversion range according to the maximum value detected by the maximum value detection circuit. In this case, the maximum value detection circuit detects the maximum value of the signal voltages output from the N difference calculation circuits or hold circuits. The A/D converter sets the A/D conversion range according to the maximum value detected by the maximum value detection circuit.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.